The movies have been conventionally shot and recorded in radiographic film as moving images (hereinafter referred to simply as “video”) at a frame rate of 24 frames/second. There are various techniques producing visual effects such as slow-motion, quick-motion in the movies. Those techniques reproduce videos that have been shot with the time base extended or shortened. The slow motion or quick motion is carried out in this way: moving images are shot at a frame rate of greater or smaller than 24 frames/second, then reproduced at a frame rate of 24 frames/second.
Assume that videos shot at 48 frames/second are reproduced at 24 frames/second, it means that videos shot in one second is reproduced in two seconds, so that slow motion with the time base doubled can be reproduced. Assume that videos shot at 12 frames/second are reproduced at 24 frames/second, it means that videos shot in two seconds are reproduced in one second, so that quick motion with the time base halved can be reproduced.
The movies can be shot, recorded, and reproduced in another way, i.e. an electronic technique has been introduced to them and it is now widely used. The electronic technique, in general, employs pickup elements such as CCD (charged coupled device) for converting videos into electrical signals, which are then recorded in magnetic recording media before they are reproduced by a reproducing device. In the case of using the film, if a shoot results in failure, the film cannot be used anymore as long as videos are recorded therein. However, in the case of using the electronic technique for shooting and recording, unnecessary video signals in recording media can be overwritten by new video signals for recording, so that the media can be reused. The shooting cost can be thus kept down.
A technique about a device which carries out the quick motion and slow motion through the electronic technique discussed above is disclosed in, e.g. Japanese Patent Unexamined Publication No. 2002-152569. A user of this device can set a frame rate before shooting. The video signals shot at this given frame rate are converted into the rate of 60 frames/second and then recorded. At the reproduction, the frames only obtained by the shooting are extracted and converted into the rate of 24 frames/second again. The slow motion or quick motion is thus reproduced.
Recently a request has arisen that an amount of recording a movie shot be minimized through the electronic technique discussed above. Because the cost involved in the shooting should be reduced, or the video signals shot are preferably stored in a recording medium having rather smaller memory capacity such as a semiconductor memory.
The conventional method discussed above converts video signals shot at lower than 60 frames/second into 60 frames/second for recording, so that the number of frames recorded could be greater than the frames actually shot. As a result, a greater capacity is required.
A moving-image compressing method (hereinafter referred to simply as “video coding”), which compresses inter-frames, is available for reducing an amount to be recorded. MPEG is an example of the video coding. The video coding that compresses inter-frames needs an equal number of frames per unit-time both in recording and reproducing, so that the video coding technique is hard to be employed in the system where different frame rates are used in reproducing slow motion and quick motion respectively.
The conventional technique discussed previously records the video signals of 60 frames/second, so that a frame rate at the shooting is restricted to not greater than 60 frames/second.